Frequency tuning system and method for finding a global optimum

ABSTRACT

A generator and method for tuning the generator are disclosed. The method includes setting the frequency of power applied by the generator to a current best frequency and sensing a characteristic of the power applied by the generator. A current best error based upon the characteristic of the power is determined, and the frequency of the power at the current best frequency is maintained for a main-time-period. The frequency of the power is then changed to a probe frequency and maintained at the probe frequency for a probe-time-period, which is less than the main-time-period. The current best frequency is set to the probe frequency if the error at the probe frequency is less than the error at the current best frequency.

PRIORITY

The present application for patent claims priority to ProvisionalApplication No. 61/733,397 entitled “STEALTHY FREQUENCY TUNING ALGORITHMCAPABLE OF FINDING A GLOBAL OPTIMUM” filed Dec. 4, 2012, and assigned tothe assignee hereof and hereby expressly incorporated by referenceherein.

FIELD OF THE INVENTION

This invention relates generally to power supplies for plasma processingapplications, and more particularly to systems and methods for frequencytuning power supplies.

BACKGROUND OF THE INVENTION

Frequency tuning in RF generators is often used to reduce reflectedpower. A typical set-up is shown in FIG. 1. Typically, but not always,some type of matching network is used to match the load to thegenerator. By correct design of the matching network (either internal tothe generator or external as shown in FIG. 1), it is possible totransform the impedance of the load to a value close to the desired loadimpedance of the generator (either at the RF output connector, typically50Ω, or at the active devices internal to the generator, typically somelow complex impedance such as 8+j3Ω) at some frequency in the range offrequencies that the generator can produce. The measure of how close theload impedance is to the desired impedance can take many forms, buttypically it is expressed as a reflection coefficient

$\rho = \frac{Z - Z_{0}}{Z + Z_{0}^{*}}$where ρ is the reflection coefficient of the impedance Z with respect tothe desired impedance Z₀ and x* means the complex conjugate of x. Themagnitude of the reflection coefficient, |ρ|, is a very convenient wayof expressing how close the impedance Z is to the desired impedance Z₀.Both Z and Z₀ are in general complex numbers.

Frequency tuning algorithms and methods try to find the optimalfrequency of operation. Optimality is often defined as the frequencywhere the magnitude of the reflection coefficient with respect to thedesired impedance is the smallest. Other measures may be minimumreflected power, maximum delivered power, stable operation etc. On atime-invariant linear load, many algorithms will work well, but ontime-varying and/or nonlinear loads special techniques are required toensure reliable operation of the tuning algorithm.

Assuming that the optimum frequency of operation is the frequency atwhich the load reflection coefficient magnitude is at its minimum, it isnoted that the relationship between the controlled variable (frequency)and the error is frequently not monotonic and furthermore the optimumpoint of operation is generally at a point where the gain ([change inerror]/[change in frequency]) is zero. To add to the challenges it isalso possible that local minima may exist in which any control algorithmcan get trapped. FIG. 2A shows a plot of load reflection coefficient ona load reflection coefficient chart (Smith chart) at the top, and FIG.2B shows the magnitude of the load reflection coefficient used as theerror as a function of frequency. This plot demonstrates the problemsdescribed above with a local minimum at f₀ separated from the globaloptimum at f_(b) by a region of high load reflection coefficient aroundf_(a) and (as is invariably the case) zero slope of the error functionat the global optimum frequency f_(b).

Two common problems on plasma loads are the nonlinear nature of the load(the load impedance is a function of power level) and that the loadimpedance changes over time (e.g., because of changing chemistry,pressure, temperature etc. over time). Another problem that is unique toplasma (or plasma-like) loads is that the plasma can extinguish if thedelivered power to the plasma falls below some value for a long enoughtime. The frequency-tuning algorithm can therefore not dwell at afrequency where enough power cannot be delivered for very long or theplasma may extinguish.

SUMMARY

Illustrative embodiments of the present disclosure that are shown in thedrawings are summarized below. These and other embodiments are morefully described in the Detailed Description section. It is to beunderstood, however, that there is no intention to limit the disclosureto the forms described in this Summary or in the Detailed Description.One skilled in the art can recognize that there are numerousmodifications, equivalents, and alternative constructions that fallwithin the spirit and scope of the disclosure as expressed in theclaims.

According to one aspect, a method for tuning a generator is provided.The method includes setting the frequency of power applied by thegenerator to a current best frequency and sensing a characteristic ofthe power applied by the generator. A current best error is thendetermined based upon the characteristic of the power, and the frequencyof the power is maintained at the current best frequency for amain-time-period. The frequency of the power is changed to a probefrequency and maintained at the probe frequency for a probe-time-period,which is less than the main-time-period. The current best frequency isset to the probe frequency if the error at the probe frequency is lessthan the error at the current best frequency.

According to another aspect, a generator is provided. The generator mayinclude a controllable signal generator to generate a frequency inresponse to a frequency control signal and a power amplifier to generatepower at the generated frequency. An output line of the generator iscoupled to the power amplifier, and a sensor is coupled to the poweramplifier to provide an output signal indicative of an impedancepresented to the power amplifier. A controller provides the frequencycontrol signal to the controllable signal generator in response to theoutput signal from the sensor, and the controller includes a processorand a non-transitory, tangible computer readable storage medium encodedwith processor readable instructions for adjusting the frequency controlsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system schematic showing generator delivering power to aload through a matching network;

FIGS. 2A and 2B depict general behavior of load reflection coefficientas a function of frequency;

FIGS. 3A and 3B depict general behavior of load reflection coefficientas a function of frequency overlaid with open loop constant powercontours of a typical RF generator;

FIGS. 4A and 4B depict general behavior of load reflection coefficientas a function of frequency overlaid with open loop constant powercontours of a RF generator with matched source impedance;

FIG. 5 is a flowchart depicting an exemplary method that may betraversed in connection with the embodiments disclosed herein;

FIG. 6 includes graphs depicting exemplary frequencies that may beprobed in connection with the method described with reference to FIG. 5and corresponding error values;

FIG. 7 is a diagram depicting an embodiment of a generator;

FIG. 8 is a diagram depicting an exemplary embodiment of the balancedamplifiers shown in FIG. 7; and

FIG. 9 is a diagram depicting a control system that may be utilized torealize the controller depicted in FIG. 7.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

Embodiments of the current invention solve the problem of finding aglobal optimum to the tuning problem without extinguishing a plasmaload. The problem can be understood by referring to FIGS. 2A and 2B. Asis evident from FIGS. 2A and 2B, any algorithm that searches for a localminimum of the load reflection coefficient will move towards the minimumfrequency, f₀, if the current frequency is between f₀ and the frequencywhere the load reflection coefficient is highest, f_(a). This situation,where the current frequency will move towards a local minimum, which isnot the desired operating frequency, is quite common. In plasma systemsin particular, the plasma chamber without a lit (ignited) plasma has avery different behavior than the chamber with a lit plasma. If thefrequency where the plasma can be ignited is between f₀ and f_(a), thenthe initial frequency will be between f₀ and f_(a). Once the plasma islit, the problem becomes how to find the global best frequency, f_(b),starting from a frequency between f₀ and f_(a). Unlike other loads,simply sweeping the frequency from f₀ to f₁ until the globally optimumfrequency, f_(b), is found is not an option. The problem is that whenthe frequency is in the vicinity of f_(a) almost no power can bedelivered to the plasma and the plasma will very likely extinguish. Ifthe plasma extinguishes, the sweep will then continue with an unlitplasma with completely different characteristics and the global optimumf_(b) will not be found unless the plasma somehow reignites in thevicinity of f_(b). Even if the plasma somehow reignites and the sweep isthus successful, the very act of allowing the plasma to extinguishduring the sweep is unacceptable in most applications.

To understand the problem, note that in order not to extinguish theplasma, the time spent probing a frequency can typically be no longerthan a few tens of microseconds. If the load reflection coefficient atthe frequency being probed is high and more than a few tens ofmicroseconds are spent at that frequency, the plasma can extinguish. Atthe same time, the time that it takes the power control system of thegenerator to adjust to the desired power level is typically on the orderof a few hundreds of microseconds, so for all practical purposes thereflection coefficient of the load is measured at the same power controlinput to the power amplifier with the actual power determined by theload impedance.

In the prior art it is known that a table of frequencies and associatedreflection coefficients is compiled by probing to find the bestoperating frequency. Compiling such a table (e.g., as described in U.S.Pat. No. 7,839,223, which is incorporated herein by reference) isdifficult because each candidate frequency may have to be visitedmultiple times until the load reflection coefficient is measured at thedesired power level. The reason why the load reflection coefficient mustbe measured at the correct power level is due to the nonlinear nature ofthe load and can be understood by referring to FIGS. 3A and 3B.

Referring to FIGS. 3A and 3B, if the generator is operating at 700 W ata frequency f_(a) and probes the frequency space with the control to thepower amplifier remaining at the current setting, the generator wouldfind the apparent best reflection coefficient at a frequency f_(c).However, as FIG. 3B shows, the actual best operating frequency is f_(b).More damaging, if the generator were to change its operating frequencyto f_(c), then once the control system adjusts the power back to thedesired setpoint (presumably 700 W or higher), the load reflectioncoefficient may be higher than at the original frequency, f_(a).Moreover, for the generator to be operating at f_(a) at 700 W generallymeans that either the setpoint for the generator is 700 W and thegenerator is capable of meeting the setpoint while applying power intothe mismatched load impedance, or the setpoint for the generator ishigher than 700 W but the generator can only deliver 700 W into themismatched load. In either case, once the frequency is changed to f_(c),it is likely that the generator will only be capable of delivering lesspower than what it could deliver at f_(a). This can result in the plasmaextinguishing if the frequency is changed to f_(c). Thus it may beconcluded that for a typical generator where, for a fixed control inputto the power amplifier, maximum power is delivered to an impedance otherthan a matched load (typically 50Ω), the procedure described in U.S.Pat. No. 7,839,223 is advisable.

However, when the frequency-probing algorithm is combined with a poweramplifier with a source impedance matched to the nominal load impedance(typically 50Ω) the algorithm can be simplified. To understand why,reference is made to FIG. 4. Assuming that the generator is operating atfrequency f_(a) at a power level of 300 W. If the probing algorithmfinds a frequency, f_(probe), at which the reflection coefficient islower than at f_(a), it also means that if the generator were to simplystay at this frequency, f_(probe), the output power from the generatorwill be higher than the power at f_(a) until the control loop of thegenerator adjusts the power back down to the setpoint. This is sobecause for a matched source impedance generator, output power increasesif the control input to the power amplifier of the generator is heldconstant and the load reflection coefficient is decreased. Thus, in thecase of a generator with a matched source impedance, there is no need todo multiple probes of the same frequency, each time adjusting thecontrol input to the power amplifier. Instead of building a table, thegenerator can simply switch to operation at the probed frequency whenthe load reflection coefficient at the probed frequency is lower thanthe current frequency since the generator can deliver at least as muchpower at the new frequency as at the old.

To describe the algorithm the following variables are defined:

-   -   f_(start): start frequency    -   f₀: minimum frequency    -   f₁: maximum frequency    -   e_(main) error at current best frequency    -   f_(main): current best frequency    -   t_(rain): time that the generator stays at current best        frequency    -   t_(probe): time that the generator takes to probe a frequency    -   f_(probe): probe frequency

Referring next to FIG. 5, it is a flowchart depicting a method forfrequency tuning. While referring to FIG. 5, simultaneous reference ismade to FIG. 6, which includes graphs depicting exemplary frequencies(and corresponding error values) that may be probed in connection withthe method described with reference to FIG. 5. As depicted, thefrequency of the generator is initially set to a start frequency,f_(start), which is a frequency at which the plasma may be ignited(Block 500). An error is then determined. (Block 502), and the currentbest frequency (e.g., f_(main1)) is initially set to the start frequency(f_(start)) while the error at the current best frequency (e.g.,e_(main1)) is set to the error determined at Block 502. In severalembodiments, the error is a measure of how close the impedance presentedto the generator is to a desired impedance (e.g., 50Ω). For example, theerror may be calculated as a load reflection coefficient magnitude,voltage standing wave ratio, reflected power, and a deviation frommaximum delivered power. And in other embodiments, the error may be avalue representative of an instability. It is contemplated that othervalues may be calculated or measured and utilized as an error value.

As depicted in FIG. 5, the generator then stays at the current bestfrequency (e.g., f_(main1)) for a main-time-period t_(main) (Block 504)before switching to a probe frequency (e.g., f_(probe1)) (Block 506),and the generator remains at the probe frequency for a probe-time-period(t_(probe)) (Block 508). In some embodiments, the probe-time-period(t_(probe)) at Block 508 is less than 100 microseconds, and in otherembodiments the probe time period (t_(probe)) is less than 10% of timethat the generator stays at current best frequency (t_(main)) at Block504.

If the probe error (e.g., e_(probe1)) at the probe frequency (e.g.,f_(probe1)) is lower than the current best error (e.g., e_(main1))(Block 510), the generator sets the current best frequency (f_(main)) tothe probe frequency (f_(probe)) (Block 512). The current best error(e_(main)) is then determined at the new current best frequency(f_(main)) (Block 502) and the process is then repeated. As depicted, ifthe probe error (e.g., e_(probe1)) at the probe frequency (e.g.,f_(probe1)) is not less than the current best error (e.g., e_(main1))(Block 510), the generator frequency is set again to the current bestfrequency (e.g., f_(main1)) (Block 514), and the process is thenrepeated. FIG. 6 depicts exemplary behavior in which two probefrequencies (f_(probe1) and f_(probe2)) are attempted (at times t₁ andt₂) before the probe error (e_(probe3) at time t₃) is lower than thecurrent best error, and then the error is reduced again at time t₄ at anew probe frequency f_(probe4) that becomes and remains the current bestfrequency through two subsequent frequency probes (f_(probe5) andf_(probe6)) that result in corresponding errors (e_(probe5) ande_(probe6)) that are greater than the error (e_(probe4)) at the currentbest frequency f_(probe4).

The choice of probe frequencies depends on the application, but toensure that the entire frequency range is evaluated, an initial sweepshould cover the entire frequency range of the generator in frequencysteps small enough to ensure that minima in the error are not missed byjumping over areas of minimum error. After an initial sweep a smallerrange around f_(main) can be probed to refine the tuning. Refining ofthe range can be repeated until the best operating frequency isdetermined with sufficient accuracy. [

The tuning algorithm may be augmented by conditions for starting andstopping the tuning algorithm. For example, a lower and upper target forthe error as well as a time to get to the lower target is typically set.The tuning algorithm will then attempt to get to the lower target in theallotted time. If it reaches the lower target the algorithm stops, andif the allotted time is exceeded, the algorithm stops if the error isless than the upper target. Once the algorithm is stopped, it isgenerally re-started when the upper target is exceeded. If the algorithmfails to reach the upper or lower targets, errors and warnings may beissued to the system controller.

Referring next to FIG. 7, it is a block diagram depicting components ofan exemplary embodiment of a generator. As shown, the generator includesone or more DC power supplies 702 that receive AC power and produce DCpower to power a radio frequency (RF) power amplifier 704 and acontroller 706. The controller 706 in this embodiment includes afrequency tuning component 708 that provides, responsive to an outputsignal 714 from a sensor 716 that is indicative of an impedancepresented to the power amplifier 704, a frequency control signal 710 tosignal generator 712. In response, the signal generator 712 generates aparticular frequency (e.g., the current best frequency (f_(main)) andprobe frequencies (f_(probe))) corresponding to the frequency controlsignal 710, and the power amplifier 704 amplifies the output of thesignal generator 712 to generate output power 718 at the particularfrequency.

FIG. 8 depicts an exemplary balanced amplifier that may be utilized inconnection with realizing the balanced amplifiers depicted in FIG. 7.

Referring next to FIG. 9, it depicts an exemplary control system 900that may be utilized to implement the controller 706 and user interfacesdescribed with reference to FIG. 7. But the components in FIG. 9 areexamples only and do not limit the scope of use or functionality of anyhardware, software, firmware, embedded logic component, or a combinationof two or more such components implementing particular embodiments ofthis disclosure.

Control system 900 in this embodiment includes at least a processor 901such as a central processing unit (CPU) or an FPGA to name twonon-limiting examples. The control system 900 may also comprise a memory903 and storage 908, both communicating with each other, and with othercomponents, via a bus 940. The bus 940 may also link a display 932, oneor more input devices 933 (which may, for example, include a keypad, akeyboard, a mouse, a stylus, etc.), one or more output devices 934, oneor more storage devices 935, and various non-transitory, tangibleprocessor-readable storage media 936 with each other and with one ormore of the processor 901, the memory 903, and the storage 908. All ofthese elements may interface directly or via one or more interfaces oradaptors to the bus 940. For instance, the various non-transitory,tangible processor-readable storage media 936 can interface with the bus940 via storage medium interface 926. Control system 900 may have anysuitable physical form, including but not limited to one or moreintegrated circuits (ICs), printed circuit boards (PCBs), mobilehandheld devices, laptop or notebook computers, distributed computersystems, computing grids, or servers.

Processor(s) 901 (or central processing unit(s) (CPU(s))) optionallycontains a cache memory unit 902 for temporary local storage ofinstructions, data, or processor addresses. Processor(s) 901 areconfigured to assist in execution of non-transitory processor-readableinstructions stored on at least one non-transitory, tangibleprocessor-readable storage medium. Control system 900 may providefunctionality as a result of the processor(s) 901 executing instructionsembodied in one or more non-transitory, tangible processor-readablestorage media, such as memory 903, storage 908, storage devices 935,and/or storage medium 936 (e.g., read only memory (ROM)). For instance,instructions to effectuate one or more steps of the method describedwith reference to FIG. 5 may be embodied in one or more non-transitory,tangible processor-readable storage media and processor(s) 901 mayexecute the instructions. Memory 903 may read the instructions from oneor more other non-transitory, tangible processor-readable storage media(such as mass storage device(s) 935, 936) or from one or more othersources through a suitable interface, such as network interface 920.Carrying out such processes or steps may include defining datastructures stored in memory 903 and modifying the data structures asdirected by the software.

The signal input component 950 generally operates to receive signals(e.g., digital and/or analog signals) that provide information about oneor more aspects of the RF power output 718. For example, the RF sensor716 may include voltage and/or current sensors (e.g., VI sensors,directional couplers, simple voltage sensors, or current transducers)that provide analog voltage signals, which are received and converted todigital signals by the signal input component 950.

The signal output component 960 may include digital-to-analog componentsknown to those of ordinary skill in the art to generate the frequencycontrol signal 710 to control the frequency of the signal generated bythe signal generator 712, which may be implemented by any of a varietyof signal generators known to those of skill in the art. For example,the frequency control signal 710 may be a voltage that is varied toeffectuate (via the signal generator 712) the frequency changes that aremade to tune the generator as described with reference to FIG. 5.

The memory 903 may include various components (e.g., non-transitory,tangible processor-readable storage media) including, but not limitedto, a random access memory component (e.g., RAM 904) (e.g., a static RAM“SRAM”, a dynamic RAM “DRAM, etc.), a read-only component (e.g., ROM905), and any combinations thereof. ROM 905 may act to communicate dataand instructions unidirectionally to processor(s) 901, and RAM 904 mayact to communicate data and instructions bidirectionally withprocessor(s) 901. ROM 905 and RAM 904 may include any suitablenon-transitory, tangible processor-readable storage media describedbelow. In some instances, ROM 905 and RAM 904 include non-transitory,tangible processor-readable storage media for carrying out the methodsdescribed herein.

Fixed storage 908 is connected bidirectionally to processor(s) 901,optionally through storage control unit 907. Fixed storage 908 providesadditional data storage capacity and may also include any suitablenon-transitory, tangible processor-readable media described herein.Storage 908 may be used to store operating system 009, EXECs 910(executables), data 911, API applications 912 (application programs),and the like. Often, although not always, storage 908 is a secondarystorage medium (such as a hard disk) that is slower than primary storage(e.g., memory 903). Storage 908 can also include an optical disk drive,a solid-state memory device (e.g., flash-based systems), or acombination of any of the above. Information in storage 908 may, inappropriate cases, be incorporated as virtual memory in memory 903.

In one example, storage device(s) 935 may be removably interfaced withcontrol system 900 (e.g., via an external port connector (not shown))via a storage device interface 925. Particularly, storage device(s) 935and an associated machine-readable medium may provide nonvolatile and/orvolatile storage of machine-readable instructions, data structures,program modules, and/or other data for the control system 900. In oneexample, software may reside, completely or partially, within amachine-readable medium on storage device(s) 935. In another example,software may reside, completely or partially, within processor(s) 901.

Bus 940 connects a wide variety of subsystems. Herein, reference to abus may encompass one or more digital signal lines serving a commonfunction, where appropriate. Bus 940 may be any of several types of busstructures including, but not limited to, a memory bus, a memorycontroller, a peripheral bus, a local bus, and any combinations thereof,using any of a variety of bus architectures. As an example and not byway of limitation, such architectures include an Industry StandardArchitecture (ISA) bus, an Enhanced ISA (EISA) bus, a Micro ChannelArchitecture (MCA) bus, a Video Electronics Standards Association localbus (VLB), a Peripheral Component Interconnect (PCI) bus, a PCI-Express(PCI-X) bus, an Accelerated Graphics Port (AGP) bus, HyperTransport(HTX) bus, serial advanced technology attachment (SATA) bus, and anycombinations thereof.

Control system 900 may also include an input device 933. In one example,a user of control system 900 may enter commands and/or other informationinto control system 900 via input device(s) 933. Examples of an inputdevice(s) 933 include, but are not limited to, a touch screen, analpha-numeric input device (e.g., a keyboard), a pointing device (e.g.,a mouse or touchpad), a touchpad, a joystick, a gamepad, an audio inputdevice (e.g., a microphone, a voice response system, etc.), an opticalscanner, a video or still image capture device (e.g., a camera), and anycombinations thereof. Input device(s) 933 may be interfaced to bus 940via any of a variety of input interfaces 923 (e.g., input interface 923)including, but not limited to, serial, parallel, game port, USB,FIREWIRE, THUNDERBOLT, or any combination of the above.

Information and data can be displayed through a display 932. Examples ofa display 932 include, but are not limited to, a liquid crystal display(LCD), an organic liquid crystal display (OLED), a cathode ray tube(CRT), a plasma display, and any combinations thereof. The display 932can interface to the processor(s) 901, memory 903, and fixed storage908, as well as other devices, such as input device(s) 933, via the bus940. The display 932 is linked to the bus 940 via a video interface 922,and transport of data between the display 932 and the bus 940 can becontrolled via the graphics control 921.

In addition or as an alternative, control system 900 may providefunctionality as a result of logic hardwired or otherwise embodied in acircuit, which may operate in place of or together with software toexecute one or more steps of the method described with reference to FIG.5. Moreover, reference to a non-transitory, tangible processor-readablemedium may encompass a circuit (such as an IC) storing instructions forexecution, a circuit embodying logic for execution, or both, whereappropriate. The present disclosure encompasses any suitable combinationof hardware in connection with software.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method for tuning a generator, the methodcomprising: providing a power amplifier within the generator that has asource impedance of 50Ω; coupling the generator to a plasma chamber,igniting a plasma in the plasma chamber, wherein the plasma has anominal load impedance of 50Ω and the impedance of the plasma varies inresponse to a power level applied to the plasma chamber; setting afrequency of power applied by the generator to a current best frequency;sensing a characteristic of the power applied by the generator;determining a current best error based upon the characteristic of thepower; maintaining the frequency of the power at the current bestfrequency for a main-time-period; changing the frequency of the power toa probe frequency; maintaining the frequency of the power at the probefrequency for a probe-time-period, wherein the probe-time-period is lessthan the main-time-period; setting the current best frequency to theprobe frequency, without changing the frequency of the power from theprobe frequency, if the error at the probe frequency is less than theerror at the current best frequency.
 2. The method of claim 1, whereinthe error is a measure of how close an impedance presented to thegenerator is to a desired impedance.
 3. The method of claim 2, whereinthe error is a load reflection coefficient magnitude calculated withrespect to a desired impedance.
 4. The method of claim 3, wherein thedesired impedance is 50 Ω.
 5. The method of claim 1, wherein the powerapplied by the generator is applied to a plasma load.
 6. The method ofclaim 1, wherein the probe-time-period is less than 100 microseconds. 7.The method of claim 1, wherein the probe-time-period is less than tenpercent of the main period of time.
 8. The method of claim 1 includingmatching a source impedance of the generator with a nominal loadimpedance.
 9. A generator comprising: a controllable signal generator togenerate a frequency in response to a frequency control signal; a poweramplifier to generate power at the generated frequency, the poweramplifier having a source impedance of 50Ω; an output line coupled tothe power amplifier; a sensor coupled to the power amplifier, the sensorgenerates an output signal indicative of an impedance presented to thepower amplifier; a controller that is coupled to the sensor and acontrollable frequency driver, the controller provides the frequencycontrol signal to the controllable signal generator in response to theoutput signal indicative of the impedance presented to the poweramplifier, the controller including a processor and a non-transitory,tangible computer readable storage medium encoded with processorreadable instructions for adjusting the frequency control signal, theinstructions including instructions to: setting the frequency controlsignal to a current best level so the frequency driver generates acurrent best frequency; determining a current best error based upon theoutput signal from the sensor; maintaining the frequency control signalat the current best level for a main-time-period; changing the frequencycontrol signal to a probe level so the signal generator generates aprobe frequency; maintaining the frequency control signal at the probelevel so the signal generator maintains the probe frequency for aprobe-time-period, wherein the probe-time-period is less than themain-time-period; setting the current best frequency to the probefrequency, without changing the frequency from the probe frequency, ifthe error at the probe frequency is less than the error at the currentbest frequency.
 10. The generator of claim 9, wherein the error is ameasure of how close the impedance presented to the generator is adesired impedance.
 11. The generator of claim 10, wherein the error is aload reflection coefficient magnitude.
 12. The generator of claim 11,wherein the load reflection coefficient magnitude is calculated withrespect to an impedance of 50Ω.
 13. The generator of claim 9, whereinthe probe-time-period is less than 100 microseconds.
 14. The generatorof claim 9, wherein the probe-time-period is less than ten percent ofthe main-time-period.
 15. The generator of claim 9, wherein the poweramplifier includes a balanced amplifier.